252 



PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



weight which it had raised. India-rubber possesses properties very 

 analogous to those of muscular tissue, in regard to the transformation 

 of heat into power. Take a strip or cord of non- vulcanized rubber and 

 attach to it a weight. It stretches, a negative power is developed, and 



Fig. 9. 



Shomng the effect of heat on mnsciilar contraction. (Tlio figure should he read from 

 riglit to left.) The first trace caused by the shock is quite high. The second is not 

 so high, but this is partly due to the point from which the tracing starts, being more 

 clovatetl, as "vrell as the horizontal lino from which the mark of the shock extends. 

 This elevation of the point of departure proves that the muscle was in a state of con- 

 traction from the influence of heat. The same effect is more and more evident up to the 

 fifth tracing, when the muscle is cooknl, and in its return to its original length the 

 tracinjifs of the shocks rejraiu their normal size. 



conformably to the mechanical theory of heat you can perceive a notable 

 increase of temperature in the rubber. Conversely, submit the rubber 

 to an increase of temperature, and it will be seen to contract and lift the 

 weight. But under these conditions the amount of i)ower produced by 

 the rubber is very suiall. It may, however, be rendered greater. About 

 two years ago. Dr. lianvier gave me an account of the following experi- 

 ment: He stretchetl a piece of rubber uatil it was fifteen or twenty 

 times as long as before, and thus produced in it a passive state, in 

 which it remained elongated eveu when traction was no longer exerted 

 upon it. While in this state, if the rubber was touched with a warm 

 body, it became considerably thickened at the pcint of contact by the 

 contraction of the part, and returned to its original leugth and thick- 

 ness. Placed in the hollow of the hand the passive thread twisted like 

 a worm, and resumed its normal length and size in a few moments. 

 Part of the phenomena exhibited in the experiment of Dr. Eanvier was 

 readily explained. The heat applied to the rubber was transformed 

 into power by it ; but what was the passive state into v/ hich the rubber 

 had been brought before the efiect above described could have been 

 liroduccd? In repeating the experiment and considering the facts more 

 attentively, I soon perceived that the duration of the traction to which 

 percha was submitted played an important part in the production of the 

 passive state. If I stretched the thread to twenty times its origiual 

 length and instantly relaxed it, it readily returned to its original 

 condition ; but if the contraction was prolonged thirty seconds, a minute, 

 or longer, the thread left to itself contracted only partially. It became 

 l)artly passive, and much more completely so when the traction was 

 more i)rolonged. jSTow, the influence of the duration of the traction can 



